Solid state current regulators are commonly used in conjunction with power supplies to protect electrical circuits and devices coupled thereto. Such regulators are commonly use in circuit interruption tripping systems for 3-phase power lines. In this application, a solid state current regulator controls the amount of current provided from the 3-phase power line to the system power supply. As the magnitude of the power in the line increases beyond a prescribed limit, the current regulator responds by shunting excess current from the line to system ground.
Known current regulators may be categorized as being either a linear shunt type regulator or a switched shunt type regulator. A typical linear shunt type regulator includes a resistor and zener diode arranged in series between the current line and ground with their interconnection controlling the base of a P-N-P Darlington transistor. When the current magnitude in the line exceeds the breakdown voltage of the zener diode, the Darlington transistor is enabled, and the excess current from the current line is shunted through the Darlington transistor to ground. This type of regulator is useful and desirable when the current level in the line is low. However, it consumes a significant amount of power, due to the regulated voltage across the transistor, when the current level in the line is high. In applications where significant levels of heat cannot be dissipated or where space or cost does not accommodate heat sinking devices, this type of shunting arrangement is unacceptable.
Switched current shunting arrangements typically involve a more complex arrangement of circuitry to enable the current path to be shunted from the line to ground. For example, in U.S. Pat. No. 4,809,125, by Matsko et al., a power supply circuit employs a custom design IC (integrated circuit) for monitoring the current level of the line and controlling the bias on a transistor which is selectively enabled to shunt excess current from the line to ground. Another known type of switched current shunting arrangement employs a comparator and a voltage divider circuit at an input of the comparator for determining when to enable a transistor which shunts the excess current from the line to ground.
While switched current shunting arrangements may be controlled so that they do not exhibit the heat dissipation problem (familiar to linear shunt type regulators), they are also not without fault. One of most significant problems associated with a switched current shunting arrangement is its tendency to generate unacceptable levels of noise. This can and does significantly impair circuit operation in certain applications. For instance, in a circuit interruption tripping system using current transformers to sense and induce the current from the line, a switched current shunting arrangement can affect the electromagnetics of the current transformers and cause them to misrepresent the true current levels in the line. Under certain conditions, the electromagnetic flux in the sensor will collapse, inducing a sensor current to the extent that the tripping system will determine that a fault has occurred and command an interruption.
Ideally, a switched current shunting arrangement in a circuit interruption tripping system couples the current transformers to a relatively low voltage and it accumulates a high level of energy quickly at power-up so that the tripping solenoid can promptly be energized after the detection of a spurious fault condition. Coupling the current transformers into a relatively high voltage will adversely affect the electromagnetics of the current transformers, as described above, and failing to quickly accumulate high levels of energy at start-up prevents the trip unit from promptly removing the fault from the system. Known trip-unit power supply arrangements have compromised on these ideals by accumulating, over an excessively long time period, large amounts of energy at lower levels, and inhibiting the trip unit from promptly removing the fault from the system.
Accordingly, there is a need for a power supply which overcomes the above problems associated with the prior art.